A polymer electrolyte fuel cell system includes a Membrane and Electrode Assembly (hereinafter referred to as MEA) structured by holding and sandwiching a solid polymer electrolyte membrane between an anode and a cathode. A fuel cell in which liquid fuel is directly supplied for the anode is referred to as a direct fuel cell. When the fuel cell generates electricity, the fuel is supplied to the anode at first. The supplied fuel is decomposed on catalysts supported on the anode and generates protons, electrons, and intermediate products. Generated cations move to the cathode side after passing through the solid polymer electrolyte membrane and generated electrons move to the cathode side through an external load. Electricity is generated by a reaction where the protons and electrons react with oxygen in the air at the cathode to produce reaction products.
For example, in a direct methanol fuel cell (hereinafter referred to as a DMFC) directly using an aqueous methanol solution as the liquid fuel, a reaction shown by a following formula (chemical formula 1) is caused in an anode and a reaction shown by a following formula (chemical formula 2) is caused in a cathode.CH3OH+H2O→CO2+6H+6e−  (Chemical formula 1);6H+6e−+3/2O2→3H2O  (Chemical formula 2);
Nowadays, research and development of the fuel cell system as a power source for various types of electronic devices, particularly in a mobile device, have been advanced, since the fuel cell system using the liquid fuel can easily allow reduction in size and weight. Here, in a case of using the fuel cell system as a power source of an electronic device such as a personal computer (PC), an output of the single MEA is small and a required electric voltage may not be obtained. For this reason, a plurality of unit fuel cells is electrically connected and used (hereinafter, a minimum unit in the power generation of the fuel cell system is referred to as a unit fuel cell and an aggregate of the unit fuel cells is referred to as a fuel cell stack).
The fuel cell stack is sometimes used with being stored in a chassis. In a case where the fuel cell system is stored in the chassis, moisture produced at the cathode as shown in the chemical formula 2 may be condensed in a narrow space between the chassis and the fuel cell system. When a surface of the cathode is fully covered with condensation water, the flooding which lowering the output may occur. In the unit fuel cell where the flooding has occurred, an electric current is forcibly passed under a condition where oxidant gas (the air) is not enough, resulting in destruction of the MEA. On the other hand, in a case where the MEA is dried too much, an ion conductivity is lost and thus power generation efficiency may deteriorate. Accordingly, in the fuel cell system, it is desired to provide a technique for managing the condensation water and keeping appropriate humidity.
Meanwhile, as the fuel cell system composed of the plurality of the unit fuel cells, it is known that there are a bipolar type fuel cell system stacking the plurality of the unit fuel cells in a thickness direction and a planar stack type fuel cell system arranging the plurality of the unit fuel cells in plane.
As for the management of the condensation water in the case of the bipolar type fuel cell system, several reports exist. As a technique for preventing the flooding, Japanese Laid-Open Patent Application JP-P2004-185935A discloses that a partition plate having a size sufficiently able to cover an entire gas passage is provided in a supplying manifold for distributing oxidant gas to the unit fuel cell and a drain receiver for receiving water condensed on the partition plate is provided under the partition plate.
In addition, Japanese Laid-Open Patent Application JP-P, Heisei 05-283094 discloses a fuel cell characterized in that an anode and a cathode are connected by a water passage and at least a portion of the water passage contacting to a water generation electrode is composed of materials having moisture osmosis.
In addition, Japanese Laid-Open Patent Application JP-P2005-322595A discloses that, when a supplying gas passage and an exhausting gas passage provided in a separator stacked on an electrode are not communicated with each other, a member of the separator forming an intermediate between the supplying gas passage and the exhausting gas passage is porous.
In addition, Japanese Laid-Open Patent Application JP-P2000-164229A discloses a technique for preventing a cathode from being dried. In JP-P2000-164229A, a polymer electrolyte fuel cell system characterized by including: a fuel cell stack of the polymer electrolyte fuel cell using a solid polymer membrane as an electrolyte; and temperature and humidity exchange means adapted to perform temperature exchange and humidity exchange by contacting reacted gas which passed a cell reaction portion with unreacted gas which will pass the cell reaction portion via a water-retentive porous material, wherein the polymer electrolyte fuel cell system is structured so that at least one of the oxidant gases can pass in a gas supplying path of a mesh form which is at least single-layered provided so as to contact to the porous material.
In addition, Japanese Laid-Open Patent Application JP-P2004-241367A discloses a fuel cell which includes a MEA and a separator and in which a reaction gas passage is formed on a surface of the separator facing the MEA, wherein a porous portion is formed in at least a part of the separator and a cooling gas passage is formed on a back surface of the reaction gas passage of the porous portion.
The techniques disclosed in the respective documents mentioned above relate to the bipolar type fuel cell system In a case of a device intended to be carried such as a laptop computer, the planar stack type fuel cell system is more suitable for the case as compared to the bipolar type fuel cell system due to a restriction of a thickness.
As the planar stack type fuel cell system, a system disclosed in Japanese Laid-Open Patent Application JP-P2004-14149A is given for example. That is, JP-P2004-14149A discloses a liquid fuel cell including: a positive electrode for reducing oxygen; a negative electrode having a hydrogen storage material; an electrolyte layer arranged between the positive electrode and the negative electrode, liquid fuel solving metal hydride; and a liquid fuel storage portion for storing the liquid fuel. The positive electrode, the negative electrode, and the electrolyte layer constitute an electrodes-and-electrolyte-combined assembly. A plurality of the electrodes-and-electrolyte-combined assemblies is arranged on an identical plane. Each of the electrodes-and-electrolyte-combined assemblies is electrically connected in series. The liquid fuel storage portion is separated every electrodes-and-electrolyte-combined assemblies by partition walls.
In the planar stack type fuel cell system, a plurality of unit fuel cells is arranged on an identical plane and the adjoining unit fuel cells are electrically connected by a power collector, thus a high electric voltage and an output can be obtained. When the planar stack type fuel cell system is employed, it is preferable that the entire fuel cell system is so small as to be fitted on a footprint of a portable device. For that purpose, it is required to constantly supply oxygen to the cathode in a manner, for example, that the fuel cell stack is mounted in a chassis and the air is forcibly supplied to a space between the fuel cell stack and the chassis by using a compact fan, or that a surface of the cathode is exposed to the atmosphere and the cathode is allowed naturally aspirating the air.
However, the naturally-aspirated structure in which the surface of the cathode is exposed to the atmosphere cannot generate electricity when the surface of the cathode is coated, thus it is difficult to employ a structure housing the fuel cell system itself in the portable device. In addition, even when the chassis only intended to house the fuel cell is separately installed, it is required not to close air holes provided in the chassis.
On the other hand, since electric power can be stably generated in a method for housing the fuel cell system itself in a chassis and forcibly blowing air with a compact fan unless an aspirating portion and an exhausting portion are closed, the method has an advantage to be employed as an electric power source of the portable device.
In the above-mentioned planar stack type fuel cell system which blows air with a fan, a technique for disposing of by-product materials is disclosed in Japanese Laid-Open Patent Application JP-P2005-129261A. Specifically, JP-P2005-129261A discloses that an adsorbing filter for adsorbing the by-product materials from the cell or a decomposing treatment filter for decomposing the by-product materials is provided to an exit portion of an air electrode (cathode) chamber in the planar stack type fuel cell system.
However, all the documents mentioned above do not disclose the method for managing the condensation water in the planar stack type fuel cell system.